1. The Field of the Invention
The present invention relates to microporous materials, and in particular, to microporous materials formed of a polymer incorporating a nucleating agent and methods for making the same.
2. The Prior Art
Microporous films or membranes have a structure that enables fluids to flow through them. The effective pore size is at least several times the mean free path of the flowing molecules, namely, from several micrometers down to about 100 Angstroms. Such sheets are generally opaque, even when made from an originally transparent material, because the surfaces and internal structure scatter visible light.
Microporous membranes or films have been utilized in a wide variety of applications, such as for the filtration of solids, for the ultrafiltration of colloidal matter, as diffusion barriers or separators in electrochemical cells, in the preparation of synthetic leather, and in the preparation of cloth laminates. The latter utilities require, of course, permeability to water vapor but not liquid water when preparing such materials as synthetic shoes, raincoats, outer wear, camping equipment such as tents, and the like. Moreover, microporous membranes or films are often utilized for filter cleaning antibiotics, beer, oils, bacteriological broths, as well as for the analysis of air, microbiological samples, intravenous fluids, vaccines, and the like. Microporous membranes or films are also utilized in the preparation of surgical dressings, bandages, and in other fluid transmissive medical applications.
Microporous membranes or films may be laminated to other articles to make laminates having particular utility. Such laminations may include a microporous layer and an outer shell layer to provide a particularly useful garment material. Further, the microporous films or membranes may be utilized as a tape backing to provide such products as vapor transmissive wound dressings or hair set tapes.
One patent relating to the preparation of microporous materials is assignee's U.S. Pat. No. 4,539,256 (Shipman), issued Sept. 3, 1985, which discloses a method of making a microporous material comprising the steps of melt blending crystallizable thermoplastic polymer with a compound which is miscible with the thermoplastic polymer at the melting temperature of the polymer but phase separates on cooling at or below the crystallization temperature of the polymer, forming a shaped article of the melt blend, cooling the shaped article to a temperature at which the polymer crystallizes to cause phase separation to occur between the thermoplastic polymer and the compound to provide an article comprising a first phase comprising particles of crystallized thermoplastic polymer in a second phase of the compound, and orienting the article in at least one direction to provide a network of interconnected micropores throughout. Brief mention is made of nucleating agents.
The use of nucleating agents to improve the optical properties of crystalline olefinic polymers is discussed in the article: C. Charles Carroll, "A Nucleating Agent for Crystalline Olefinic Polymers," Modern Plastics, September 1984, pages 108-112. U.S. Pat. Nos. 3,207,735; 3,207,737; 3,207,738; and 3,299,029 also disclose the usage of nucleating agents in polymer crystallization techniques.